1. Field of the Invention
The present invention relates to an olefin polymerization catalyst, which comprises a transition metal compound, a modified clay compound and an organic aluminum compound, and to a process, for producing olefin polymers, which employs the catalyst.
2. Description of the Related Art
The use of catalyst systems comprising combinations of transition metal compounds and organic metal compounds is already known in methods of producing polyolefins by olefin polymerization. Kaminsky et al. have disclosed in Japanese Unexamined Patent Publication No. 58-19309 that catalysts employing metallocene and methylaluminoxane exhibit high activity for the production of olefin polymers such as propylene.
However, although the catalyst system disclosed in this publication is excellent for polymerization activation, because the catalyst system is soluble in the reaction system it is common to employ a solution polymerization system which, however, not only restricts the production process but also necessitates the use of large quantities of relatively costly methylaluminoxane in order to produce polymers with industrially useful properties. Consequently, the problems of cost and high levels of aluminum residues in the polymers have to be dealt with when employing such catalyst systems.
A catalyst system wherein the aforementioned soluble catalyst system is borne on an inorganic oxide carrier such as silica is disclosed in Japanese Unexamined Patent Publication No. 60-35006. However, the polymer activity per amount of methylaluminoxane has still been insufficient even when the olefins are polymerized by the process described here.
Methods for improvement include those disclosed in, for example, Japanese Unexamined Patent Publications No. 4-8704, No. 4-11604, and No. 4-213305, wherein catalyst systems prepolymerized with a small amount of methylaluminoxane are used for gas phase polymerization to obtain polymers with excellent polymerization activity and satisfactory particulate form. However, despite the advantage of a reduced amount of methylaluminoxane, the polymerization activity has still been less than satisfactory, and therefore a higher activation of catalyst systems has been a desired goal.
Also, Japanese Unexamined Patent Publication No. 1-503788 describes a method for producing ethylene/.alpha.-olefin copolymers by a high pressure/high temperature process employing as the catalyst a transition metal compound and aluminoxane. This method, however, presents problems regarding the use of the catalysts on a large industrial scale, given the difficulty of synthesizing methylaluminoxane in a reproducible manner as mentioned above, and the fact that in spite of the high cost of methylaluminoxane, the amount of the methylaluminoxane used must be increased considerably with respect to the transition metal compound in order to achieve sufficient activity.
Recently, new promoter catalysts have been studied which do not employ organic aluminum oxy compounds such as methylaluminoxane, and for example, special boron compounds have been disclosed as effective promoter catalysts in Japanese Patent National Publications No. 1-501950 and No. 1-502036. Nevertheless, because these boron compounds are highly complex compounds, the problem of cost has not been resolved.
Japanese Unexamined Patent Publications No. 5-301917 and No. 7-309907 disclose catalyst systems employing inexpensive clay or clay treated with inorganic salts. The olefin polymerization activity in these catalyst systems, however, has been less than adequate.
Also, Japanese Unexamined Patent Publication No. 7-224106 teaches that a high activity catalyst system can be obtained by using an interlayer cation modified clay prepared by reacting clay with an organic cation, for the purpose of actively utilizing the surface negative charge of the clay minerals as a promoting catalyst; however, in terms of ash content or process applicability when considering actual production, it has been desirable to develop catalyst systems with even higher activity and better polymerization behavior, which can be used at high temperature and in solution polymerization processes wherein the residence time of catalysts is longer. It has also been desired for the interlayer cation-modified clay in such catalysts to have better storage stability.